Impact testing appears as a most promising tool for gaining information on coating behavior in load-bearing applications. During dynamic impact test an indenter impacts successively the surface of the coating with constant force and frequency. The deformation of the coated specimen during impact testing is affected by the mechanical properties of both the substrate and the coating. Varying the impact load and the number of impacts, the evolution of coating surface deformation and contact fatigue failures can be observed. In the paper, the influence of dynamic impact load and number of impacts on the resulting impact crater volume and morphology is analysed, and the interpretation of the results in form of Wohler-like dependance is suggested and demonstrated on two types of HVOF sprayed Co-based alloy coatings. The low-number impact craters evolution and subsurface cracks propagation of HVOF sprayed Co-based alloy coatings is analyzed in more detail, by means of 3D optical microscopy and SEM. The results showed, that the higher ability to deform plastically increased the coatings dynamic impact fatigue lifetime. The cracks, responsible for coatings destruction, spread predominantly along the intersplat boundaries in the pile-up area.

A computational fluid dynamics model for understanding the HVAF process and the influence of the process parameters on the particle flight properties is investigated. Achieving this objective involves a novel approach to modeling the HVAF process with pressure inlet boundary conditions and integration of the mixing chamber. The study comprises the prediction of the flow fields described by a set of equations consisting of continuity, momentum, energy, and species transport. These equations are then solved with realizable k-ε turbulence model, a two-step chemistry model and eddy dissipation model to simulate the combustion reaction. Consequently, the interaction between the CoNiCrAlY alloy particles and the flow is modeled using a Lagrangian approach considering the forces acting on the particles and the heat transfer. The results show that the combustion chamber pressure is mainly affected by the compressed air and propane parameters. Furthermore, the flight behavior of the smaller particles is significantly influenced by the gas flow, while the larger particles tend to maintain their momentum and energy. Through the simulation model, an in-depth process understanding of the HVAF process can be achieved. More importantly, the model can be used as a tool for efficient process development.

As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800 °C.

The plasma torch design affects the particle-plasma interaction, in-flight properties and the coating microstructure. When spraying metallic powders, the in-flight oxidation as well as the particle velocity and temperature determine the mechanical, corrosion and oxidation properties, which have a major impact on the in-service degradation of bond coats. This study aims to determine the microstructural and mechanical properties of as-sprayed CoNiCrAlY coatings deposited on the Inconel 718 alloy. Depositions were made using a High Velocity Plasma Spray Process (HVPS), which is based on a special plasma torch design. In-flight particle characteristics were determined to elucidate the kinetic and thermal regime of HVPS process.

The erosion behaviour at room temperature (RT) of as-deposited SPS, EB-PVD and APS YSZ-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50 μm alumina and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via SEM and XRD. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-sprayed and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada (NRC).

In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

Corrosion resistance of coatings deposited by thermal spraying technology HVOF (High Velocity Oxygen Fuel) requires high density in coating and good adhesion to substrate material. The majority of thermally sprayed materials meet the requirements of high corrosion resistance in terms of their composition. However, porous structure raises doubts about the performance of thermally sprayed coatings regarding sufficient protection to the base material. In fact, corrosion protection is a basic coating function. However, , no sufficient attention has been dedicated to the issue of component protection against corrosion attack using HVOF sprayed coatings. In this study, NiCoCrAlY, NiCoCrAlTaReY, NiCoCrAlYHfSi, and CoCrAlYTaCSi coatings were deposited on the substrate material 1.4923. The coatings were deposited using HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The coatings were exposed to the corrosive-aggressive environment in the form of molten salts mixture with composition of 60 % V 2 O 5 and 40 % Na 2 SO 4 at the selected temperature of 750 °C. Further, all coatings were exposed to cyclic conditions. Weight changes of individual specimens were measured after every cycle and results were recorded in diagrams. After the corrosion test, all evaluated coatings were analyzed using scanning electron microscope (SEM), analysis of elemental composition (EDS) and X-Ray diffraction. The NiCoCrAlY and NiCoCrAlTaReY coatings showed the best corrosion protection in selected corrosive aggressive environment, forming the protective oxide layer that prevented further corrosion attack. On the contrary, NiCoCrAlYHfSi and CoCrAlYTaCSi coatings were found not to be suitable for corrosion protection of components working in selected corrosive environment.

Stellite 6 is a widely applied hardfacing alloy with good corrosion and wear resistance. Several different techniques have been used to deposit this alloy, like oxyacetylene, Tungsten Inert Gas welding overlay, CO 2 laser cladding and thermal spray methods. This paper evaluates the wear and corrosion resistance of Stellite 6 coatings applied by high velocity oxygen fuel (HVOF) spraying and gas tungsten arc welding (GTAW) hot wire technique. Mechanical properties and microstructural characteristics of the coatings are assessed and compared. The influence of these properties in the wear results is further discussed.

The study of both the interface strength and residual stresses within a plasma sprayed ceramic coating is of great interest which main purpose is a better understanding of the mechanical properties of metal/ceramic systems. In this work, experiments involving a LASAT facility (LASAT: Laser Shock adhesion test) were implemented in order to analyse the adhesion and the damaging behaviour (debonding and buckling) of alumina coatings onto Co-based alloy. Similar alumina coatings were deposited using same plasma parameters with various surface preparations: smooth or severe grit blasting, with and without pre-oxidation. The non-destructive analyses (Optical and IR imaging) of the buckled region after LASAT have allowed to compare and discuss the interface strength of the studied coated samples. Further discussion was carried out by analysing the blister, resulting from the release of residual stresses within the coating after LASAT. It was thus evidenced that the residual stress state is a key parameter on resulting adhesive properties. This explorating work suggests using the LASAT method to analyse the adhesion and residual stresses within thermal sprayings.

A Vickers indentation method has been applied to determine the interfacial fracture toughness of modern multilayer thermal barrier coatings. The delamination behavior of four types of coating systems will be discussed and compared with results based on modified four-point bending (4PB) tests. The investigated multi-layer coating system consists of a CoNiCrAlY-bond coat applied via low-pressure plasma spray (LPPS) on a nickel-based superalloy and an atmospheric-plasma sprayed (APS) top layer of type gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ). A conventional YSZ mono-layer system is used for reference. The effects of GZO and YSZ microstructure were investigated using top coats with low and high porosities for both (multi- and single-layer) coating systems. Isothermal oxidation tests at 1100 °C up to 500 hours were performed to study the interaction between thermal aging and fracture behavior. Investigations of microstructure and sintering behavior show a significant influence of the annealing conditions on fracture toughness. It has been observed, that with increasing annealing time, the stiffness and thus the crack driving force of the GZO layer is increased due to sintering effects and healing of submicron defects. The lower stiffness and higher defect density of GZO seem to be the main reason for the reduced fracture toughness of the YSZ / GZO interface compared to the YSZ / CoNiCrAlY interface. As a result, the delamination of the top coat is observed to shift from the top coat / bond coat interface into the top coat double-layer.

Due to good performance in abrasive and sliding wear and enhanced oxidation behavior, coatings based on Co-Cr-W alloys are widely used in industrial applications, where the material is exposed to high temperature. Within the scope of this study, a Co-based alloy similar to commercial Stellite 6, which additionally contains 20.6 wt.% of vanadium, was deposited by Twin Wire Arc Spraying (TWAS). Multi-criteria optimization using statistical design of experiments (DoE) have been carried out in order to produce adequate coatings. The produced coatings have been analyzed with respect to their tribological behavior at elevated temperatures. Dry sliding experiments were performed in the temperature range between 25°C and 750°C. Oxide phases were identified in the investigated temperature range by X-ray diffraction (XRD) using synchrotron radiation. The V-doped Stellite-based coating possesses a reduced coefficient of friction (COF) of about 0.37 at elevated temperatures (above 650°C), which was significant lower when compared to conventional Stellite 6 coating that serves as reference. In contrast, both produced coatings feature a similar COF under room temperature. X-ray diffraction reveals the formation of cobalt vanadate and vanadium oxides above 650°C. The formation of vanadium oxides exhibits the ability of self-lubricating behavior, thus leading to enhanced tribological properties.

The CoCrAlSiY alloy powder with Si mass concentrations of 0, 2% and 5% was prepared in this work. The oxidation kinetics curves of all three kinds of powders after 300 h oxidation at 1000 °C were plotted. In addition, the phase constitution of alloy powder and the distribution of β phase were analyzed by SEM and EDS. Furthermore, the effect of Si-addition on the melting temperature and oxidation resistance of the alloy powder were investigated by DSC-TG from the room temperature to 1400 °C. And the element concentrations at the grain boundary of alloy powder with Si addition of 2% were also analyzed. The results show that the melting temperature of alloy powder decreases as increasing Si content, which indicates that adding Si element could influence on the selective oxidation of Al and Cr elements in the alloy system, and improve the oxidation resistance of CoCrAlY powder. In addition, the weight gain of powder with Si addition of 2% is lowest. And Si element has a enrich tendency in the grain boundary. Therefore, the higher Si content would have a negative effect on the high temperature oxidation resistance of powder.

Cr 3 C 2 -25%NiCr, Stellite 6, NiCrBSi and Hastelloy C-276 coatings were deposited on substrate material P91 by HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The resistance against high temperature corrosion was evaluated exposition of coatings to corrosive-aggressive environment in the form of molten salts mixture with composition of 60% V 2 O 5 and 40% Na 2 SO 4 at temperature of 750 °C. Further, coatings were exposed to cyclic conditions. After the corrosion tests, all coatings were analyzed using scanning electron microscope (SEM), and analysis of elemental composition (EDX). Alloys-based coatings showed very similar corrosion mechanism in the selected aggressive environment and the same can be stated about cermet coatings. The obtained results prove that HVOF deposited coatings can replace current surface protection of components in power equipment such as nitriding.

Wear-resistant cobalt–based alloy (Stellite 12) coatings deposited by plasma transferred arc (PTA), commonly used to protect critical mechanical components in harsh environments, were modified by addition of hard ceramic particles (TiC) and solid lubricant compounds (MoS 2 and CaF 2 ) to improve the overall tribological performance. In this preliminary study, microstructural, microhardness and tribological analyses were carried out to assess: a) the feasibility of PTA deposition of thermally sensitive phases characterised by very low density; b) the effect of the addition of a mixture of soft and hard phases on the coating hardness; c) the effect of the modified composition in terms of wear resistance; d) the effect of the addition in terms of lubrication (friction coefficient and produced heat). Results showed that: a) an appropriate pre-consolidation of feedstock materials can be effective in preserving the heat-sensitive phases within the microstructure of PTA deposits; b) the addition of a total amount of 5% wt. of solid lubricants and reinforcing carbides produced a limited decrease in the coating hardness (about 13%) and an evident improvement in terms of friction coefficient but, on the other hand, a remarkable reduction (about 30%) in wear resistance. Further investigation will be addressed to optimize the composition of modified feedstock to counteract the softening effect of lubricant phases without depressing the self-lubrication behaviour.

A computer-controlled laser test rig (using a CO2 laser) offers an interesting alternative to traditional flame-based thermal gradient rigs in evaluating thermal barrier coatings (TBCs). The temperature gradient between the top and back surfaces of a TBC system can be controlled based on the laser power and a forced air back-face cooling system, enabling the temperature history of complete aircraft missions to be simulated. An air plasma spray (APS) deposited TBC was tested and, based on experimental data available in the literature, the temperature gradients across the TBC system (ZrO2-Y2O3 YSZ top coat / CoNiCrAlY bond coat / Inconel 625 substrate) and their respective frequencies during air-to-air combat missions of fighter jets were replicated. The missions included (i) idle/taxi on the runway, (ii) take-off and climbing, (iii) cruise trajectory to rendezvous zone, (iv) air-to-air combat maneuvering, (v) cruise trajectory back to runway and (vi) idle/taxi after landing. The results show that the TBC thermal gradient experimental data in turbine engines can be replicated in the laser gradient rig, leading to an important tool to better engineer TBCs.

Kennametal’s UltraFlex surface treatments provide wear and corrosion resistant coatings in locations that are not easily protected by other methods such as weld overlay or thermal spray. The UltraFlex surface treatments are based on a powder metallurgy process where coatings are applied to substrates in a “green” state and then heated to fuse the coating into a dense, uniform, metallurgically bonded layer with almost no porosity. Unlike most weld or spraying processes, the process for applying UltraFlex does not require direct line-of-sight with the substrate, so components with very complex geometries, difficult to reach locations, or small IDs can be protected with a high-quality surface. UltraFlex coatings are available in multiple grades which are selected depending on the specific application environment. UltraFlex tungsten carbide grades have excellent abrasion and erosion resistance and cobalt alloy grades have excellent corrosion and high temperature resistance.

This paper presents the results of a study of the oxidation behavior of thermal barrier coating (TBC) with air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900 ¢XC and evaluated using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and transmission electron microscopy (TEM). The thermally grown oxide (TGO) developed in TBC with LPPS bond coat was composed of only α-Al 2 O 3 and the TGO developed in TBC with CS bond coat is composed of α-Al 2 O 3 and γ-Al 2 O 3 . Despite the presence of this metastable γ phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between γ-Al 2 O 3 and the formation of mixed oxides was investigated through the measurement of γ- Al 2 O 3 thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the γ- Al 2 O 3 thickness ratio. Overall findings indicate that the oxidation behavior of the TBC with CS bond coat is superior compared to that of the TBC with LPPS bond coat.

In this study, the effects of bond coat compositions with and without 0.5 mass.% Y on oxidation behaviour were investigated. Oxidation behaviour was accessed in terms of The TGO structure and growth kinetics. After isothermal oxidation tests, the TGO formed on Y-free CoNiCrAl coatings was composed mainly of alumina without the presence of Y-rich oxide and showed lower growth kinetics than that formed on conventional CoNiCrAlY coating. This result indicates that potential improvements to the bond coat oxidation behaviour can be achieved using Y-free bond coat compositions.

This work compares the oxidation behaviour of CoNiCrAlY coatings manufactured by the HVOF, APS and CGDS processes when submitted to temperatures of 1000°C and 1100°C. The as-sprayed coating microstructural features were characterised using SEM and XRD analysis before being subjected to isothermal heat treatments in an air furnace. Oxide scale composition was determined using XRD, SEM and EDS analysis while the oxide growth rates were obtained using mass gain measurements. The as-sprayed HVOF and CGDS coatings exhibit similar microstructures while the APS samples have a significantly higher porosity and oxide content levels. Results from the oxidation experiments indicated that the oxide growth rate of HVOF and CGDS were lower than that of the APS samples. The results also indicated that samples oxidised at 1100°C have a lower oxide growth rate than those oxidised at 1000°C. Analysis of the oxidation process up to 100 hours indicates that the formation of dense α-Al 2 O 3 is more favourable at 1100°C while a transition alumina, θ-Al 2 O 3 is more favourable at 1000°C. Furthermore, the surface profile of the samples oxidised at 1100°C were more uniform and free of protrusions.

The excellent corrosion resistance and biocompatibility of titanium make of it the material to choose for biomedical applications. Cold spraying, as a new coating technique, can be used to deposit protective Ti coatings onto less performing materials such as stainless steel and Co-Cr alloys, commonly used for biomedical implants. In addition, Cold Spray has the advantage, in comparison with conventional thermal spray techniques, to permit the deposition of oxygen-sensitive materials. In this study, Cold Sprayed Ti coatings were prepared on Co-Cr alloy substrates by using different spray process conditions. The microstructure of coatings was observed by SEM and the inner porosity was estimated by image analysis. Oxygen and nitrogen contents were investigated on a set of free standing deposits obtained using different process parameters. In the same way, the roughness and microhardness of deposits, such as the adhesion strength with the substrate, were measured. Finally, the corrosion performance of the coatings was evaluated by mean of open circuit potential measurement (OCP) and potentiodynamic polarizations scans. The electrochemical response was therefore discussed and compared to the corrosion behaviour of the Co-Cr alloy substrate and the bulk Titanium.